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Abstract: The work shows, for the first time experimentally and numerically, how the thermal emissivity spectra of a Mg-rich olivine (i.e. Mg1.84Fe0.16SiO4, Fo92Fa8, with Fo=Mg2SiO4 and Fa=Fe2SiO4) significantly change as a function of temperature. Thermal infrared spectra at two different temperatures between about 350 and 770K were collected in order to simulate the typical diurnal equatorial temperature variation of the surface of Mercury. Such temperature increase (i.e. 420K) modifies the olivine spectrum shifting in wavelength its emissivity features, simulating the spectrum of an olivine strongly enriched in iron (a Fa abundance increase from 8% to 23%). This “chameleon-like behavior” indicates that the spectroscopic investigations of the surface of Mercury must take into account the local solar time dependency to properly infer the compositions of planetary materials by remote sensing acquisition of future space missions. Although previous works already investigated the effect of temperature on IR olivine spectra, none of them definitively pointed out the reasons of the above mentioned spectral changes. [Copyright &y& Elsevier]

Ureilites meteorite fragments present different levels of shock classified on the basis of optical observations of shock features in silicates. We have investigated by scanning electron microscopy (SEM), micro X-Ray Diffraction (XRD) and Micro-Raman Spectroscopy (MRS) five ureilitic fragments (FRO 95028, FRO 01089, FRO 97013, FRO 01088 and FRO 01012) with increasing degree of shock (from S2 to S6) with the purpose to observe if there is any correlation between the level of shock recorded by silicates and the presence of the different carbon polymorphs formed by shock. XRD patterns showed that all investigated samples contain nano-graphite. Besides this phase, sample FRO 95028, with S2 degree of shock, contains nanodiamond, while samples from level of shock S3 to S6 contain both nano- and micro-diamond. XRD results support the shock formation of micrometer-diamonds found in FRO 01089, FRO 97013, FRO 01088 and FRO 01012 with the assistance of (Fe, Ni)-alloys as catalysts at pressure >10 GPa (S3 shock level recorded by silicates). The formation of polycrystalline diamond is already allowed at pressure between 5-10 GPa. Temperature estimated by a graphite-thermometer based on MRS data, provided values in the range of 1291 to 1398 °C ±120 °C, revealing that there is not a considerable variation of the graphite temperature with the increasing degree of shock. • The silicates shock features and the carbon polymorphs present some correlation. • Polycrystalline diamonds are already allowed to form at 5–10 GPa. • Micro diamonds are only present in samples with shock levels from S3 to S6. • The graphite temperature and shock features in silicates are not correlated. • The shock event(s) is the formation process of ureilitic nanographite and diamond. [ABSTRACT FROM AUTHOR]

When and how Earth's ancient crust – the cratons – became underpinned by cool, thick lithospheric mantle roots capable of hosting diamonds are among the most controversial aspects of Archean geology. Alluvial diamonds in cratonic sedimentary cover rocks, whose minimum age is determined by detrital-zircon geochronology, provide a unique perspective on this topic. A new discovery of a diamond-bearing quartz-pebble conglomerate from the northern Slave craton, Canada contains detrital zircon with a restricted U-Pb age distribution that has a dominant peak at ∼2.94 Ga and depositional age of ∼2.83 Ga. Pressure-temperature constraints derived from an olivine-diamond host pair lie on a conductive Mesoarchean geotherm of ∼36–38 mW/m2, comparable to the coolest modern lithospheric geotherms. This result is at odds with a hotter geothermal gradient related to nearby Mesoarchean komatiites. We propose a model whereby early building blocks for cratons were small but with deep cool roots that formed by slab-stacking, and were subsequently juxtaposed with regions of thinner, hotter lithosphere. This heterogeneous initial architecture later amalgamated and thickened through lateral accretion forming the more uniformly thick cratonic lithosphere observed today. Thermal modelling indicates that stacking/thickening of cool initial lithosphere into a lithospheric keel thick enough to stabilise diamonds is the most likely way of generating the observed geotherm by Mesoarchean times. • Detrital zircon in host sedimentary rock has a Mesoarchean U-Pb age of >2.83 Ga. • Oxidizing diamond-forming fluids had crustal component (non-mantle δ 13 C- δ 15 N). • PT constraints show cool conductive Mesoarchean mantle geotherm of ∼36–38 mW/m2. • Early cratonic building blocks were small with deep cool roots. • These blocks likely formed through slab-stacking. [ABSTRACT FROM AUTHOR]

The study of diamond and its solid inclusions is of paramount importance to acquire direct information on the deepest regions of the Earth. However, although diamond is one of the most studied materials in geology, the diamond-inclusion relationships are not yet understood: do they form simultaneously (syngenesis) or are inclusions pre-existing objects on which diamond nucleated (protogenesis)? Here we report, for the first time, adhesion energies between diamond (D) and forsterite (Fo) to provide a crucial contribution to the syngenesis/protogenesis debate. The following interfaces were investigated at quantum-mechanical level: (i) (001) D /(001) Fo , (ii) (001) D /(021) Fo , and (iii) (111) D /(001) Fo . Our data, along with the ones recently obtained on the (110) D /(101) Fo interface, revealed an unexpected thermodynamic behaviour, all interfaces showing almost equal and low adhesion energies: accordingly, diamond and olivine have an extremely low chemical affinity and cannot develop preferential orientations, even during an eventual epitaxial growth. Combining these results with those of our previous work concerning the morphology constraints of diamond on its inclusions, we can state that the two main arguments used so far in favour of diamond/inclusions syngenesis cannot be longer considered valid, at least for olivine. [ABSTRACT FROM AUTHOR]

• The Moncuni (Lanzo Massif, W-Alps) ophiolite experienced subduction to 600 C ∘ and 2.1 GPa at dry conditions. • Ductile flow at temperatures > 700 C ∘ prior to subduction. • Seismic fracturing at eclogite-facies conditions recorded by pseudotachylyte. • Preservation of pre-subduction assemblages and pseudotachylyte glass during subduction. • The Moncuni ophiolite is the proxy for the rheology of subducting dry oceanic slab. The ophiolitic peridotite and gabbro of Moncuni (Southern Lanzo Massif, Western Alps) retain pre-subduction mantle-to-oceanic, high-temperature (> 700 C ∘) ductile fabrics. These fabrics are overprinted by seismic fracturing and faulting associated with pseudotachylytes. Within the gabbro, the pseudotachylytes preserve dry glass and pristine microlites. The occurrence of rare, minute garnet and the static development of eclogite-facies assemblages in local hydrated domains indicate that pseudotachylytes experienced subduction conditions of 600 C ∘ and 2.1 GPa. The exceptional survival of glass and the absence of post-oceanic ductile deformation demonstrate prevailing dry conditions during the entire Alpine subduction and exhumation path. Dry conditions inhibited reaction kinetics and viscous flow. In contrast, the majority of the Alpine ophiolites, derived from the upper hydrated portions of the oceanic lithosphere, show pervasive fluid-assisted metamorphism and ductile deformation. The Moncuni body can, therefore, be regarded as representative for the rheological behaviour during subduction of seismic, dry, deeper oceanic lithosphere that is rarely exhumed to the Earth's surface. In Moncuni, the brittle-ductile transition of dry oceanic rocks is constrained to be between 600 and 750 C ∘. This temperature range corresponds to the observed cut-off of intermediate-depth seismicity within subducting slabs. We infer that the base of the seismic layer corresponds to the brittle-ductile transition of a dry slab rather than the locus of antigorite breakdown triggering earthquakes by dehydration embrittlement. [ABSTRACT FROM AUTHOR]